The invention relates to apparatus for reading an optical data carrier having an optically readable pattern in substantially parallel data tracks, and more particularly to such an apparatus having a relatively movable transducer for generating a read signal, and an error signal generator providing an error signal to a correction device for reducing tangential tilt of the transducer. The invention also relates to a method of reducing tangential tilt in such an apparatus.
During reading of an optical data carrier the transducer generates a radiation beam for scanning optically readable patterns on the data carrier. As the density with which data is recorded on an optical data carrier increases the tolerance for deviations (tilt) of the angle of incidence from the perpendicular angle diminishes. The tilt can have a radial component and a tangential component. The tangential component (tangential tilt) is defined as the component of the deviation in a plane oriented parallel to the track to be read and transversely to the data carrier. The radial component (radial tilt) is the component of the deviation in a plane oriented transversely to the track to be read and transversely to the data carrier.
An apparatus of the type defined in the opening paragraph is known from EP 569 597 A1. In the known apparatus the transducer comprises a transparent plate arranged in an optical path from a radiation source to the data carrier. The orientation of the transparent plate depends on an error signal which is a measure of the radial and/or tangential tilt of the data carrier. For this purpose, in a first embodiment, a sub-beam is split off the beam which is reflected from the data carrier. The sub-beam is imaged onto a four-quadrant detector via a screen which blocks a central portion of the beam. The error signal is derived from differences between four signals generated by means of the detector. In another embodiment the apparatus is equipped with auxiliary means for determining the orientation of the transparent plate and auxiliary means for determining the orientation of the data carrier. Both auxiliary means comprise a separate radiation source and a detector for generating four signals. In this embodiment the error signal is derived from the two sets of four signals.
It is an object of the invention to provide an apparatus of the type defined in the opening part, which requires less additional optical means for generating the error signal for the purpose of tangential tilt correction.
To this end, according to the invention, the error signal generator derives the tangential tilt error signal from a temporal asymmetry of the response of the read signal to the optically readable pattern. The invention enables the error signal for the correction means to be derived from the read signal which is already generated for reproducing the data from the data carrier. As a result of this, the apparatus in accordance with the invention does not require any additional optical means.
It is to be noted that from EP 583 818 A1 a device is known for detecting distortions in a signal and correcting the distortions by means of a variable equalizer. However, said document does not describe the relationship between tangential tilt of a transducer with respect to a data carrier and the properties of the read signal. Neither is it apparent from said document how an error signal is to be generated for correction means for reducing the tangential tilt.
The apparatus in accordance with the invention is based on the following insight. In the case of a perpendicular angle of incidence of the radiation beam generated by the transducer the read signal accurately corresponds to the optically readable pattern recorded on the data carrier. For an angle of incidence which deviates from the perpendicular orientation in a tangential direction the radiation beam is imaged on the data carrier as an asymmetrical light spot. The asymmetry of the light spot gives rise to a temporal asymmetry in the response of the read signal to the optically readable pattern. The response of the read signal S0(t) to the optical pattern can be defined as
S0(t)=xcfx84.xcexa3{i=xe2x88x92M,i=M}xcex2i.x(txe2x88x92i),
where x( ) is the signal recorded on the data carrier, xcfx84 is a gain factor and xcex2i are coefficients and xcex20=1.
The following approximation applies:
S0(t)=xcfx84(xcex2xe2x88x921.x(txe2x88x921)+(1xe2x88x92xcex2xe2x88x921xe2x88x92xcex21).x(t)+xcex21.x(t+1)),
For a limited tilt angle, for example of the order of 0 tot 1xc2x0 the difference between the coefficients xcex2xe2x88x921 and xcex21 is substantially proportional to the value of the tilt angle. Since in the apparatus in accordance with the invention the magnitude of the tangential tilt is reduced it is also possible for the bit detection means to derive from the read signal S0(t) a signal S2(t) which is a reliable estimate (represented by [ ]) of the signal x(t) recorded on the data carrier.
S2(t)=[S0]≈x(t).
Given the fact that the signal recorded on the data carrier can be estimated in a reliable manner it is possible at instants at which a step appears in the recorded signal to derive an error signal indicative of the direction and magnitude of the tilt from the signal which is distorted by tangential tilt. The calculation of the error signal SE(t) is simplest for those instants t which comply with (S2(txe2x88x921), S2(t), S2(t+1))=(0,0,1) or (1,0,0). For these two situations the instantaneous values of the read signal successively comply with:
S0xe2x80x2=xcfx84.xcex21,
and
S0xe2x80x3=xcfx84.xcex2xe2x88x921.
The difference between S0xe2x80x2 and S0xe2x80x3 forms a suitable error signal for tangential tilt correction.
However, it is also possible to derive an estimate for the error signal from the read signal at instants which comply with (S2(txe2x88x921), S2(t), S2(t+1))=(0,1,1) or (1,1,0). For these two situations the instantaneous values of the read signal successively comply with:
S0xe2x80x2=xcfx84(1 xe2x88x92xcex2xe2x88x921),
and
S0xe2x80x3=xcfx84(1xe2x88x92xcex21).
Again, the difference between S0xe2x80x2 and S0xe2x80x3 forms a suitable error signal.
In another embodiment of the apparatus in accordance with the invention the error signal is calculated from a sequence of values of the read signal in the proximity of a step. This has the advantage that variations in the error signal as a result of fluctuations in the step response are reduced.
In a practical embodiment of the apparatus in accordance with the invention, the error signal generator includes an auxiliary signal generator having delay circuits for producing a multivalent signal from the read signal, a sample and hold circuit for storing a sampled value of the auxiliary signal, and a circuit for generating a control signal for the sample and hold circuit when a step in the read signal is signalled.
In a preferred embodiment the error signal generating means calculates the error signal again for every step in the read signal, as a result of which a rapid correction of the tangential tilt is achieved. One circuit signals a negative step, and another circuit, independently, signals a positive step.
The apparatus may also include channel decoding means, for example decoding means for an FLL code, for example EFM. Error detection means can be based on, for example, a Reed-Solomon code, for example the Cross-interleaved Reed-Solomon code. The use of error detection means enables errors in the estimated value S2(t) of the signal (x)t recorded on the data carrier to be corrected. This makes it possible to extend the range of the angle within which tangential tilt can be corrected.
On the one hand, the highest accuracy of the error signal is achieved if the detected step forms part of a regular pattern having a length of a few periods. However, such patterns occur relatively infrequently in the read signal. On the other hand, shorter pattern s are more frequent. Consequently, the detection of such patterns allows a more frequent adaptation of the error signal. An attractive compromise for RLL coded data is provided when a negative step in the read signal is signalled upon the occurrence of a bit sequence which successively comprises one bit of the value 0, n bits of the value 1, n bits of the value 0 and one bit of the value 1; and a positive step in the read signal upon the occurrence of a bit sequence which successively comprises one bit of the value 1, n bits of the value 0, n bits of the value 1 and one bit of the value 0. In an embodiment favorable in the case that the signal recorded on the data carrier is modulated with the EFM channel code, n=3.
Correction of tangential tilt makes it possible to increase the data density in the direction of the tracks to be read. The embodiment in which a further circuit generates a further error signal which is a measure of radial tilt allows the data density to be increased also in a direction perpendicular thereto. One embodiment uses means for the generation of the further error signal described in the Application filed simultaneously with the present Application and listed in the Cross-Reference to Related Applications. Said Application is therefore incorporated in the present Application by reference.
The invention also relates to an apparatus for reading from and writing on an optically readable data carrier. The transducer for reading and the transducer for writing can utilize common means, for example a common radiation source and/or common optical means.
In the case of preformatted data carriers an apparatus in accordance with the invention can utilize information specified in headers of the data carrier for writing on an optical data carrier. Such an apparatus in accordance with the invention can, for example, alternately write information onto the data carrier and read information, such as address information, from the headers. During reading the apparatus can generate an error signal in a manner corresponding to that in an apparatus in accordance with the invention intended exclusively for reading a data carrier.